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【单选题】
三相桥式有源逆变电路 中如果逆变角β小于换流重叠角γ,即β<γ时,晶闸管T1、T2换流还未结束,电路的工作状态到达uA与uB交点P,从P点之后,uA将高于uB ,晶闸管T2承受反压而重新关断,而应该关断的T1却承受正压而继续导通,从而造成 。A.
逆变成功
B.
逆变失败
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【单选题】 关于培养基下列说法不正确的是
A.
可分为孢子培养基、种子培养基和发酵培养基
B.
种子培养基成分要容易吸收利用，成分丰富完整，总浓度略稀薄为宜
C.
孢子培养基有机氮源要高，以促进孢子产生
D.
发酵培养基要求组成丰富完整，还需要合成产物所需的特定元素、前体和促进剂等。
【单选题】I _____ in New York for two years, but I’ve never regretted my final decision to move back to my motherland.
A.
worked
B.
was working
C.
have worked
D.
had worked
【简答题】It was the district sports meet. My foot still hadn’t healed(痊愈)from a(n) injury. I had whetherIt was the district sports meet. My foot still hadn’t healed(痊愈)from a(n)____injury. I had____whether o...
【单选题】I had to meet my mother at the railway station as she ______ before.
A.
had never been
B.
has never been
C.
has never come
D.
had never come
【单选题】I had to meet my mother at the railway station as she _______ before.
A.
had never been here
B.
has never been here
C.
has never come here
D.
had never come here
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